Generally speaking, wastewater treatment plants of the kind utilized in cities and towns in North America typically incorporate a number of tanks through which the wastewater is passed, and where various operations are performed. Virtually all such treatment plants utilize bacterial action to break down organics which the wastewater contains. The bacteria utilized are obligate aerobes, which means that there is a necessity to provide an on-going supply of oxygen to the bacteria in those tanks which are designated for the digestion or breaking down of organic materials. The organic materials and the bacteria form what is known as activated sludge, and the term "activated sludge wastewater treatment tanks", is normally applied to the containers where such digestion takes place
Depending upon design, other tanks in a typical plant may be used as settling tanks to allow separation of sludge from clarified liquor
The present invention focuses particularly upon the wastewater treatment tanks utilizing activating sludge, and it is appropriate here to discuss several of the problems associated with conventional such tanks.
The rate at which the bacteria in a wastewater treatment tank consume oxygen while they digest the organics is referred to as the "oxygen uptake rate" or O.U.R. In the monitoring procedure for a wastewater treatment plant, one of the essential factors which must be determined on a regular basis is the O.U.R. The O.U.R. provides an index of bacterial activity, and that activity can vary from time to time for several reasons. One reason relates to the amount of incoming organics at different parts of the daily cycle. At night, when most users of the wastewater system are seeeping, there is normally a drop-off in the amount of organics arriving at the activated sludge treatment tanks. In many cases, this will lead to a decrease in the O.U.R. during the nighttime hours.
Another reason for a change in the O.U.R. may be that, for some reason, bacteria have been removed from a tank faster than the natural replenishment rate of the bacterial colonies. In another example, the reason for a decrease in the O.U.R. may be the influx of toxic chemical materials (for example from industrial processes) which have an inhibiting or lethal effect on the bacteria.
With regard to the variation of incoming organics, it will be appreciated that conventional plants are not equipped to take advantage of the natural decrease during the nighttime hours, should this take place, since most typical plants are designed to blow air into the activated sludge treatment tanks at a given rate, which is higher than that calculated to provide the maximum possible oxygen demand. In other words, most typical plants have a blower system for blowing air into the activated sludge treatment tanks at a single rate, and the compressors are expected to run continuously at that rate. However, this leads to an over-oxygenation of the liquid in a treatment tank during those periods when the incoming organics are reduced substantially. It will be appreciated that it requires a substantial amount of energy to blow air into the treatment tanks, since this must be done under a head of from 5 feet to 10 feet (depending on the depth of the nozzles), and therefore the air must be moved against a specific superatmospheric pressure. The electrical energy required to move all of this air can represent a cost in the millions of dollars per year, for a given city of substantial size. A considerable saving in expenditure could be effected if the rate at which air is blown into the treatment tanks could be regulated in accordance with the amount of incoming organic material, in such a way that the oxygenation of the liquid more closely approximates the actual bacterial activity.
As previously indicated, it sometimes happens that the activated sludge in a treatment tank is removed at a rate faster than the natural regeneration rate of the bacterial colonies, resulting in a shortfall of the bacterial agent, and hence a reduction in the O.U.R. This may happen despite the fact that incoming organics are at the normal daytime level, which normally would produce a higher O.U.R. One of the responses to this situation is to return settled sludge into the system, to replenish the bacterial agent.
In view of the foregoing discussion, it is an object of an aspect of this invention to provide a method and apparatus for automatically determining the O.U.R. in an activated sludge wastewater treatment tank.
It is an object of another aspect of this invention to provide a method and apparatus for determining the O.U.R. of an activated sludge wastewater treatment tank on a regular and continuing basis, so that any substantial variations of the O.U.R. can be observed.
It is an object of a further aspect of this invention to provide a method and apparatus which utilizes a regularly determined O.U.R. to regulate the rate at which air is pumped into an activated sludge wastewater treatment tank.
Finally, it is an object of yet another aspect of this invention to provide a method and apparatus which utilizes a regularly and automatically computed O.U.R. to control the rate at which settled sludge is returned into the system.
To round out the prior art, reference may be had to the following patents:
U.S. Pat. No. 3,607,735, issued Sept. 21, 1971 to Hover et al;
U.S. Pat. No. 4,256,575, issued Mar. 17, 1981 to Garrett et al;
U.S. Pat. No. 3,909,409, issued Sept. 30, 1975 to Lange et al;
U.S. Pat. No. 4,416,781, issued Nov. 22, 1983 to Bailey et al;
U.S. Pat. No. 4,171,263, issued Oct. 16, 1979 to Roberts, et al;
U.S. Pat. No. 3,925,721, issued Dec. 9, 1975 to Petroff;
U.S. Pat. No. 3,547,811, issued Dec. 15, 1970 to McWhirter;
U.S. Pat. No. 3,872,003, issued Mar. 18, 1975 to Walker;
U.S. Pat. No. 3,823,728, issued July 16, 1974 to Burris.